This invention relates to electrodes for alkaline storage batteries, and more specifically to a water-base process for making such electrodes whereby the electrode active material is held in a non-sintered binder of coagulated polytetrafluoroethylene (hereafter PTFE). The invention is useful for all of the popular alkaline storage battery electrodes (e.g. zinc, cadmium, nickel, silver etc.), but has particular advantage in reducing the weight and cost of nickel electrodes.
One of the major drawbacks to the more extensive use of nickel alkaline batteries (e.g. Ni-Zn) is, for example, the high cost of the nickel electrodes. Originally such electrodes used sintered carbonyl nickel plaque current collector-supports which were impregnated with nickel salts and then converted into nickel hydroxide. Typically this was accomplished by filling the pores of the nickel plaque with an aqueous solution of a nickel salt and subsequently converting the salt to the hydroxide by chemical, electrochemical or thermal processes. The process normally required several repetitions to introduce the desired amount of nickel hydroxide into the plaque and utilized unnecessarily high amounts of nickel which added considerable cost and weight to the electrodes.
More recent electrodes eliminate the plaque and bind the nickel hydroxide in a polymer binder. Some are made by milling (i.e. calendaring) nickel hydroxide, graphite, binder and a plasticizer together and then roll bonding it to a current collecting grid. Various techniques are used to make these electrodes porous. In one technique, a mixture of two immiscible thermoplastic resins is used as an initial binder, one of the resins later leached from the mass with a suitable solvent and the active material retained in a microporous matrix of the remaining insoluble resin. An additional sintering step removes any remaining soluble resin and coalesces the remaining resin.
In another process (i.e. Strier et al. U.S. Pat. No. 3,706,601), an aqueous dispersion of active material particles (i.e. ZnO) is mixed with an aqueous dispersion of a latex type polymer (e.g. PTFE), and a film cast from the mixture, which is then dried and, sintered (e.g. 260.degree. C.-375.degree. C.). The sintered film is then rolled to form a fibrous polymer structure binding the electrode active material together.
McBreen U.S. Pat. No. 4,000,005 discloses a process for making, compressed, non-sintered-binder nickel electrodes by forming a filter cake which is a coagulum of active material particles (i.e. nickel hydroxide, cobalt hydroxide and graphite) entrained in a three dimensional, reticulated, open cell polyvinylidene fluoride (i.e. Kynar) binder which was precipitated slowly out of solution in the presence of the active materials. As the polyvinylidene fluoride slowly precipitates and coagulates it entrains the active materials.
Still other proposed techniques include: (1) precipitating nickel hydroxide as a slurry from a solution of a nickel salt and vacuum impregnating a porous nickel conductor with the slurry; (2) applying a layer of an aqueous paste of nickel hydroxide, nickel powder and a binder to a metallic substrate, compressing it to remove excess water, drying it and compressing it again to achieve intimate nickel hydroxidenickel metal interfacial contact; (3) mixing nickel hydroxide, graphite, dimethylformamide, polyvinylidene fluoride and dimethylacetaminde together, casting it into a thin film (e.g. 0.7-0.8 mm), drying it for a short while, immersing it in water to coagulate the polyvinylidene fluoride, and finally wrapping it with a current collector and fabric separator to form the electrode. Still another technique involves mixing dry powdered PTFE with the active materials in a nonaqueous lubricant to form a slurry, filtering the slurry to form a filter cake containing about 25%-50% lubricant, calendering the filter cake to a self supporting condition, drying it and integrating it with a current collector.
PTFE-bound electrodes are highly resistant to the chemical and electrochemical environment of alkaline cells and, do not lose active material (i.e. graphite and nickel oxide) during formation. Hence they offer advantages over other polymer binders. However they cannot be made by the McBreen process as no acceptable solvent system is known, and even if there were the slow precipitation and cost of materials and the handling thereof in a plant make the McBreen process undesirable on a commercial scale. Moreover the sintered PTFE processes cannot be used with all active materials as the sintering temperatures (i.e. 260.degree. C.-375.degree. C.) tend to destroy the electrochemical activity of some of them notably nickel hydroxide (Ni(OH).sub.2).
Accordingly objects of the present invention are to provide an inexpensive, water-base process for manufacturing, non-sintered, PTFE-bound electrodes as well as the electrodes themselves. These and other objects of this invention will become more apparent from the description thereof which follows.